Tight oil is the abbreviation of tight reservoir oil, the main storage space of which are unconventional reservoirs such as tight sandstone, marl, dolomite or shale. At present, tight oil has been commercialized in North America and has brought great changes to the world oil and gas exploration and extraction, which is gradually affecting the pattern of world energy supply and demand. China's tight oil is mainly land-based, with great resource potential, and widely distributed, mainly in Songliao, Qaidam, Ordos and Sichuan Basin. At present, with the decreased number of newly discovered conventional oil fields and the gradual reduction of recoverable reserves of oil and gas resources, tight oils will be one of the major oil and gas resources in the future. However, the tight oil reservoir has the characteristics such as low porosity, low permeability and low pressure, accompanied by the development of natural microfractures, thus has defects such as difficult development and energy supplement, and low degree of use. At present, the volume fractured horizontal well production process is widely used in the extraction of tight oil. Due to the presence of natural microfractures, conventional waterflooding extraction can easily cause water logging of the horizontal well. Other extraction methods such as gas flooding, chemical flooding have expensive operation cost and poor extraction effect. Therefore, there is need to find out an oil reservoir extraction process which is economic and reasonable.
Huff-puff oil extraction technology is a kind of production process for performing injection and production in the same well and oil reservoir layer, which has two important oil production mechanism, with the first to enhance the stratum energy around the oil well, and the second to reduce crude oil saturation in reservoir micropores using the flooding characteristics of the injection agent. As an effective method for enhancing oil recovery, the huff-puff technology is widely used in experimental development. However, there are a variety of types of huff-puff technologies, including CO2 huff-puff, microbial huff-puff, steam huff-puff and water huff-puff, of which the adaptive reservoir characteristics and mechanism of action are not the same. Therefore, a huff-puff oil production method suitable for the extraction of tight oil must be found.
In the tight oil reservoir modified by using the volume fracturing process, under the large displacement (>6 m3/min) conditions, a large number of fracturing liquid (single-stage fracturing liquid injection volume >800 m3) is pressure injected into the reservoir, and proppants with high flow conductivity are added for crack support, forming complex, seam-type artificial fractures having flow conductivity hundreds of times larger than that of matrix. At the same time, the complex artificial fractures formed after multi-stage fracturing in the horizon section of horizon well communicate a large number of natural fractures in the reservoir, so that the number of effective fractured seepage channels for flowing the reservoir crude oil into the bottom of the well is increased, expanding the area of the seepage. Under such characteristics of the reservoir seepage, it is very easy to form gas channeling or flooding and thus reduce the oil production capacity of the oil well when the gas injection or water injection is carried out in the adjacent injection wells. However, the use of a method for enhancing oil recovery in huff-puff oil production in a same horizontal well effectively solve the problem of gas channeling or water logging; meanwhile, relying on the complex seam-type fractures in the reservoir fractures, the effective area of huff-puff is greatly expanded, and the effect of huff-puff is increased.
Huang Dazhi (Study on oil production mechanism of waterflood huff-puff. Petroleum Geology and Recovery Efficiency, 2004, 11 (5): 39-41) introduces that the huff-puff oil production by water injection refers to an oil production method, wherein water is injected into production layer, with the injected water preferentially fill favorable parts such as high porosity and high permeability zone and large pore throat or fractures; after the well is shut down, under the action of capillary force, the injected water is replaced with the crude oil in the medium, small pore throats or the matrix, resulting in redistribution of oil and water in the production layer, then the well is opened to decrease pressure, and the replaced oil is taken out together with the injected water (see FIG. 1).
The advantages of waterflood huff-puff lie in that, it can effectively improve the water logging in fractures during the waterflood extraction of fracture reservoir; meanwhile it can improve oil recovery by making full use of oil and water imbibition replacement and stratum energy supplement principle. In addition, the injection agent (injected water) is low in price, adequate in resources, and low in operating costs. For high-input-cost extraction for tight oil, in the volume fractured horizontal wells, the implementation of water injection can rely entirely on seam-type fractures, which expands the contact area of injected water and the fractures, and further increases the amount of oil and water replacement. At the same time, after alternately repeated water injection and oil production process, the swept area of oil washing and displacement multiple are increased, the efficiency of oil displacement is improved, thereby the oil recovery is enhanced.
At present, the home and abroad research on waterflood huff-puff is focused on the development of straight wells, but less directed to the waterflood huff-puff in the horizontal wells, and there is no study on huff-puff oil production of tight oil from a volume fractured horizontal well in the unconventional reservoirs. At present, the kind of reservoirs is mainly developed by natural energy failure. In this method, the energy of the stratum is reduced rapidly, the yield is decreasing rapidly, the final recovery rate is low, and the economic benefit development of the reservoir cannot be realized. Therefore, for the tight oil reservoir extraction from the fractured horizontal well, it still needs further in-depth exploration of better extraction methods. In addition, for the waterflood huff-puff technology, theoretical analysis and experiments on how the parameters such as water injection amount, water injection speed, shut-down time, oil production time, and oil production speed influence the oil production efficiency of the waterflood huff-puff have not been clearly understood.